In the fabrication of integrated circuits, it is highly desirable in certain circumstances to use an etching process having a specific selectivity and a high aspect ratio to enable a higher degree of device integration. To that extent, the use of pulse plasma (e.g., time-modulated plasma) is an emerging technology that is under active development. For instance, in forming a polycide layer in an integrated circuit, it has been suggested that improved results are obtainable by using an etching method that utilizes pulse plasma technology. Particularly, it has been found that the notches and profiled defects in a pattern may be removed by applying time-modulated radio frequency (RF) power having a step function to the plasma etching when forming the polycide layer. The RF power is usually modulated into a step function by a repetitive on/off operation. This repetitive on/off operation of the RF power generates the step function in discrete repeating periods. Thus, the use of pulse plasma etching is a generally simple operation that may significantly reduce the notching and side attack during the etching of polycide or polysilicon.
FIG. 1 illustrates an RF generating system as in the prior art for generating time-modulated RF power for use in generating pulse plasma. The RF generating system 10 comprises an oscillator 12 for generating the RF power, an RF power amplifier 14 for amplifying the RF power to a required level, and a mixer 16 interposed between the oscillator 12 and amplifier 14 for receiving an external modulation function, such as a step function. Therefore, when the signaling step function is high, the RF power is passed on to the amplifier 14, and when the step function is low, no RF power is passed on to the amplifier 14. Accordingly, the RF power signal is modulated into a periodic pulse prior to being supplied to a plasma reaction device 20.
FIG. 2 shows a general waveform of the time-modulated RF power signal that is produced by the RF generating system 10 and applied to the plasma reaction device 20 of FIG. 1. The time-modulated RF power has a function F(.omega.) that can be expressed as Equation (1) below: EQU F(.omega.t)=f(.omega..sub.0 t).multidot.g(.omega.t) (1)
where F(.omega.t) is the function representing the time-modulated RF power, f(.omega..sub.0 t) is the continuously generated RF power, and g(.omega.t) is the modulation function (i.e., the step function). The RF power generated by oscillator 12 can be expressed as Equation (2) below: EQU f(.omega..sub.0 t)=A sin .omega..sub.0 t (2)
where A represents the amplitude and .omega..sub.0 represents the angular frequency of the applied RF power. An illustration of the waveform f(.omega..sub.0 t) is provided in FIG. 3A. The modulation function g(.omega.t) can be expressed as Equation (3) below: EQU g(.omega.t)=1 (0&lt;t&lt;T.sub.1), or EQU g(.omega.t)=0 (T.sub.1 &lt;t&lt;T) EQU g.omega.(t+T)!=g(.omega.t) (3)
where T denotes the period of the modulation function, and where 0&lt;T.sub.1 &lt;T. Further, .omega. denotes an angular frequency of the modulation function. Accordingly, g(.omega.t) has the form of a step function that will turn on the RF power for a predetermined time T.sub.1, and turn off the RF power for the remainder of the time period T. An illustration of the waveform g(.omega.t) is provided in FIG. 3B.
In order to analyze the frequency response of the function F(.omega.t) of the modulated RF power, the frequency response of the modulation function g(.omega.t) is analyzed first. Assuming that the ratio of a duty cycle of the modulated function g(.omega.t) is 50%, that is, T.sub.1 =T/2, then g(.omega.t) can be expressed as the Fourier series of Equation (4) below: ##EQU1## where k is a dummy symbol, also referred to as an index of summation. When f(.omega..sub.0 t)=A sin .omega..sub.0 t, then the waveform of the time-modulated RF power can be expressed as function F(.omega.t) in Equation (5) below:
Therefore, when the RF power F(.omega.t) of Equation (5) is modulated with the modulation function g(.omega.t) of Equation (4), a series of sidebands are formed adjacent to a carrier frequency. In the present case, the frequency and amplitude of the sidebands are .omega..sub.0 .+-.(2k-1).omega. and A/.pi..multidot.1/(2k-1), respectively. This is graphically illustrated in FIG. 4, wherein the frequency spectrum distribution of the function F(.omega.t) is illustrated with EQU F(.omega.t)=A sin .omega..sub.0 t.multidot.g(.omega.t) ##EQU2## respect to the mode number k. As can be gleaned from FIG. 4, the sideband modes .omega..sub.0 .+-.(2k-1).omega. corresponding to the frequency of the modulation function g(.omega.t) are generated around the applied carrier frequency .omega..sub.0.
When the time-modulated RF power is applied to the plasma reaction device 20, a high reflective wave is typically generated because the sideband, which has a multiplicity of frequencies, is generated over a wide bandwidth at modulation frequency intervals that are near the frequency of the RF power. The high reflective wave may be undesirable because it can damage the RF generating system 10, and thereby deteriorate the stability and reproducibility of the time-modulated RF power.
In order to reduce the effects of a high reflective wave, a matching network may be used. However, a matching network is generally tuned to a specific frequency for reducing the reflective wave at that frequency alone. Thus, a matching network may not be able to simultaneously reduce the reflective wave in a plurality of frequencies over a wide band width response, such as the one shown in FIG. 4. Therefore, the amount of RF power reflected by the pulse plasma device 20 may not be significantly reduced by a matching network because of the sideband. In severe cases, 80% or more of the applied RF power may be reflected.